Electrochemical devices or systems of the type referred to herein include one or more of the metal-halogen battery systems, such as a zinc-chloride battery system. These metal-halogen battery systems generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack station typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metalhalide electrolyte. The electrolyte circulation subsystem operates to circulate the metalhalide electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte ionic components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive or "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the store temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, were it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
During the operation of the zinc-chloride battery system it is inevitable that some hydrogen is evolved from the zinc electrode substrates, since the equilibrium potential of the metallic zinc in aqueous zinc-chloride is such that hydrogen gas must be formed by the decomposition of water. While the evolution of hydrogen gas from pure zinc metal is very low due to the high over voltage for hydrogen, this evolution rate is subject to several considerations including zinc morphology and the presence of impurities in the zinc deposit. Nevertheless, this rate of hydrogen evolution is typically very small in comparison to the evolution of chlorine gas which is generated at the chlorine electrode during the charging of the battery system. While it is important in the zinc-chloride battery system to minimize the rate of hydrogen evolution due to the fact that hydrogen evolution represents a coulombic inefficiency, the amount of hydrogen gas in the battery stack can be reduced subsequent to its generation by a reaction with chlorine gas.
In the zinc-chloride battery system, a controlled chemical reaction between chlorine gas and hydrogen gas is achieved by providing one or more lights having a wave length of light radiation generally within the ultraviolet range. Thus, during the charging of the zinc-chloride battery system the chlorine and hydrogen gases generated by the cells are exposed to an ultraviolet light source which will cause these gases to form hydrogen chloride. Additionally, during the discharging of the zinc-chloride battery system the chlorine gas and any hydrogen gas which may be present in the gas space of the stack area above the cells may also be exposed to this ultraviolet light source.
In prior zinc-chloride battery stack designs, the battery cells have been housed in trays or otherwise arranged so that the tops of the cells are open in order to permit the chlorine and hydrogen gases to be liberated from the cells. However, in accordance with the present invention the battery cells are arranged in substantially closed cell compartments which include a top section for controlling both the flow of electrolyte into and out of the cell compartment and the liberation of gas from the cell compartment. With such a closed cell compartment, it is possible for a quantity of gas to be trapped in the gas space above the cells in the cell compartment during a standby mode. Since hydrogen gas will continue to evolve at the zinc electrodes of the cells in the cell compartment in the standby mode after the battery system has been charged, it is possible for a quantity of hydrogen gas to accumulate within the cell compartment over an extended period of time. Then, when the discharging of the battery system is commenced, the accumulated hydrogen gas from the cell compartment will be liberated therefrom and exposed to the ultraviolet light source in a proportion relative to the amount of chlorine gas which could exceed a desirable level.
Accordingly, it is a principal objective of the present invention to provide an improved electrochemical system battery stack design which is capable of selectively venting gas from the cells.
It is a more specific objective of the present invention to provide an improved battery stack design which is capable of automatically venting gas from the cell at predetermined times without resorting to any electrical controls.
It is a further objective of the present invention to provide an improved cell design for a zinc-chloride battery system in which hydrogen gas is vented from the cell during a charge standby mode.
To achieve the foregoing objectives, the present invention provides an improved battery stack design which features means for defining a substantially closed compartment for containing the battery cells and at least a portion of the electrolyte for the system, and means in association with the compartment means for selectively venting gas from the interior of the compartment means in response to the level of the electrolyte within the compartment means. The venting means includes a relief valve having a float member which is actuated in response to the level of the electrolyte within the compartment means. This float member is adapted to close the relief valve when the level of the electrolyte is above a predetermined level and open the relief valve when the level of electrolyte is below this predetermined level.
Additional advantages and features for the present invention will become apparent from a reading of the detailed description of the preferred embodiments which make reference to the following set of drawings in which :